Design Decisions

Chris Fernandez, Dakota Folger-Reeder, and Michael Shullick

1 On Deck

● History ● CPU Choice ● Board Layout and Architecture ● PPU ● Impact

2 ➔ 1889 Brief History of ◆ founded in Japan as a playing card Nintendo and the manufacturer ➔ 1966 NES ◆ Nintendo shifts to toys ➔ 1975 ◆ Nintendo has success with arcade games like Donkey Kong ➔ 1983 ◆ Nintendo’s Family (“Famicom”) released in Japan ◆ Video games crash in North America ➔ 1984 ◆ Nintendo releases the Advanced Video System (flopped) ➔ 1985 ◆ Nintendo released the Nintendo Entertainment System

3 NES Dissection

So what’s inside?

4 CPU - MOS 6502

● Introduced in 1975, a sixth of the cost of competitors ● Designed by same engineers of the 6800 ● Cost reduction performed by removing extra features of and use of new fabrication technology for depletion-mode MOSFETs. ● Used in many famous applications

5 6502 Specifications

● Proprietary derivative of the 6502 manufactured by ● -based with only 6 special registers ● 8-bits, little endian ● 16 bit addresses ● 1-3 MHz clock ● 56 instructions ● Fast “zero ” mode for faster access to first 256 addresses in memory, intended to be quasi-registers

6 Ricoh 2A03 - Deviations from MOS 6502

● Hardwired to disable BCD mode of 6502 ● Memory-mapped components: ○ APU ○ DMA ○ Game controller polling ● Different versions for different regions

7 Architecture

8 Memory

● Memory is split between the CPU and the PPU(Picture Processing Unit) ● CPU Memory is divided into 3 main parts, ROM in the cartridges, the CPU RAM, and I/O registers ● CPU and PPU interact only through I/O registers

9 Memory and I/O

● The memory addressing, which is the first page in memory was also mirrored three times between $0800 and $1FFF. ● If data is written to $0000 it is also written to $0800, $1000, and $1800. ● The I/O registers are located between $2000 and $401F ● There are 8 main I/O registers, with them being mirrored every 8 between $2008 and $3FFFF ● Addresses $4000 to $4013 are used for sound, and controller input

10 Game Cartridges

● Depending on the cartridge games use different amounts of the CPU memory ● If the game has 16 kB of ROM then it only uses $C000-$FFFF ● If it has 32 kB of ROM it uses the all of the assigned memory for ROM which is $8000-$FFFF

11 Registers

● CPU had three special purpose registers, and three for general purposes ● PC, SP, and Accumulator were the three special purpose registers ○ The SP did not have a stack overflow detection and would just wrap around from $00-$FF ○ The Accumulator stored the results of operations and could be used to hold values taken from memory ● Two Index registers were used for general addressing or as a counter ● Processor held 7 flags based on the result of instructions

12

● There are three types of interrupts, IRQ, Non-Maskable Interrupts (NMI) and Reset ● IRQs are generated by either Memory Mappers or by software ○ Can be ignored if the Disable Flag is set ● NMIs is generated by the PPU at the end of each frame ○ Can be ignored if bit 7 of PPU Control Register 1 is clear ● Reset interrupts are generated when the system is first turned on or the reset button is pressed

13 Picture Processing Unit (PPU)

● The custom PPU, known as the Ricoh RP2C02, was developed by Ricoh Company, Ltd. for Nintendo ● Has eight registers, visible to the CPU by its address spaces $2000-$2007 ● The PPU is responsible for interpreting data to properly display the orientation, spatial position, color, and shape of sprites ● Runs 3 times faster than the CPU clock speed NES board, PPU framed in red, CPU in blue

14 Picture Processing Unit (PPU) cont.

● The PPU sends 240 visible scan lines of sprite data to televisions to create graphical frames ● Finite set of 54 hardware controlled colors for a total of 25 concurrent colors on screen ● Able to display up to 64 moving objects on screen at a time ● Resolves at a whopping 256 × 240 pixels ● Extended capabilities through the usage of

mappers NES graphical output to TV

15 PPU Memory Designation

● PPU operates in smallest graphical unit of “tiles” ● 8 kB (2 tiles of 4 kB) of ROM/RAM allocated on the Game Pak itself for tiles data ● 2 kB of external RAM to store extra maps/nametables

● 32 bytes of internal SRAM for color data Internals of NES Game Paks ● 288 total bytes of internal DRAM for Object Attribute Memory (OAM) 16 PPU Attribute Tables

● 64- 8x8 arrays that control spatially which color pallet should be applied to the objects in the array ● Each byte in the array is responsible for a 4x4 tile (32x32 pixel) ● This means every two-bits in a byte has the information for a quadrant of a graphical tile

Attribute Table example 17 PPU Rendering Process

● Each frame is 89,342 PPU clock cycles long ● The PPU renders 262 total scanlines per frame, each lasting for a total of 341 PPU cycles ● Each rendered tile takes 4 memory accesses, each requiring 2 PPU cycles per access ● Using a combination of latches, shift registers, and counters the PPU is read in the data from memory ● This data is held in the latches until it is time to be fed to the appropriate shift registers (every 8 clock cycles)

18 PPU Timing Diagram 19 Impact & Legacy

● Poor design of front-loading cartridge ● Lockout malfunction ● Judged the second greatest console of all time by PC Magazine ● Succeeded by the Super Nintendo Entertainment System (SNES) ● NES Classic Edition released in 2016 (still unavailable everywhere)

20 References

● https://wiki.nesdev.com/w/index.php/PPU ● https://wiki.nesdev.com/w/index.php/PPU_registers ● https://wiki.nesdev.com/w/index.php/PPU_rendering ● https://en.wikipedia.org/wiki/Picture_Processing_Unit ● https://fms.komkon.org/EMUL8/NES.html ● http://nesdev.com/NESDoc.pdf

21